It has been known for some time for position finding to be carried out on the basis of radio links, for example within the satellite-based GPS (Global Positioning System). The European satellite navigation system Galileo, as well as positioning methods based on terrestrial radio sources, offer further applications. Position-finding methods such as these and corresponding appliances allow the user to determine his position by measuring the distance to a specific number of wire-free signal sources such as satellites or base stations. By way of example, each GPS and Galileo satellite transmits unique digital sequences, which include a time identification and the satellite position. The signals are normally coded with long spread codes. The spread codes for the individual satellites are virtually orthogonal with respect to one another, so that the signals can be distinguished from one another in the receiver. By way of example, the spread codes of the various GPS and Galileo satellites are synchronized with respect to one another by means of high-precision atomic clocks which are installed in the satellites.
The receiver evaluates the relative delays between the signal transmission from different radio sources (GPS satellites, Galileo satellites or terrestrial transmitters), and uses these delays to determine the so-called delay time offsets. Together with the data relating to the position and the time reference of the various radio sources, the delay time offsets can be used in order to locate the receiver exactly. The receiver for this purpose calculates the so-called pseudo ranges, which represent the distance to each radio source. Navigation software can then calculate the user position on the basis of the pseudo range to each radio source and the position of the radio sources, by solving a set of non-linear equations.
The delay time offsets between the signals received from the radio sources are frequently measured by determining a correlation maximum in a receiver architecture and by assuming that this maximum corresponds to the direct line-of-sight path (LOS) with additive white Gaussian noise (AWGN). The problem is that the individual location signals do not always reach the receiver along a direct line-of-sight path, but are often reflected on a large number of obstructions such as buildings or hills. These reflected location signals travel over a longer path than the location signal which is propagating on the line-of-sight path before they are received by the receiver, and are thus delayed. In the case of this so-called multipath signal propagation, that is to say in which various versions of the location signal arrive at the receiver at different times, the delayed location signals are superimposed at the receiver antenna. Depending on the phase offset, the location signals are subject to constructive or destructive interference. Thus, depending on the transmission scenario, the amplitude and the phase of the received signal may be very different.
The reflection, diffraction and scatter of the location signals on obstructions also leads to attenuation of the location signals. This makes it harder to identify the signal in the receiver, in the event of multipath signal propagation.
The effects that have been mentioned require high receiver sensitivity in order to make it possible to reliably identify and synchronize location signals.
Many conventional modern receivers for position-finding systems are based on first of all despreading the sample values of the received signals and then subjecting them to coherent integration and non-coherent integration. The resultant statistical values are passed to a detector, for example a Neyman-Pearson detector, which maximizes the probability of identification of the location signals in accordance with the desired requirements.
The detector compares the statistical values supplied to it with a threshold value. Provided that a statistical value is greater than the threshold value, it is assumed that a location signal has been received. In the converse situation, the received signal is not classified as a location signal. This is intended to avoid signals which are not location signals being used for position finding. Furthermore, this method also prevents location signals with an excessively low reception amplitude being used for position finding. Modern receivers frequently have detectors which are optimized for additive white Gaussian noise.